speaker

Abstract

A loudspeaker comprising: two acoustic diaphragms mounted to face in axially opposite directions; two voice coils, each having an axis and an axial length and configured to reciprocate along its axis to drive one of the diaphragms, the axes being substantially parallel and the two axes passing through the two diaphragms; at least one magnet forming part of a chassis assembly, the chassis assembly being configured to provide two axially extending gaps for reciprocating each of the voice coils within one gap, wherein the at least one magnet and the chassis assembly are adapted to cause magnetic flux to flow in opposite directions across the gaps, and wherein, when in use the diaphragms are at their predetermined maximum negative excursion, the voice coils overlap in the axial direction by between 10% and 90% of their average axial length, and wherein, when in use the diaphragms are in a relaxed position between their maximum negative excursion and maximum positive excursion, the voice coils do not overlap in the axial direction.

Description

Technical Field

[0001] The present invention relates to loudspeakers, and in particular to mirrored coaxial acoustic arrays having a nested motor structure. Background Art

[0002] The structure and operation of dynamic loudspeaker drive units are well known. A vibrating diaphragm is attached to a coil of wire known as a voice coil, and the voice coil is placed in a magnetic field, usually provided by one or more permanent magnets. By passing an alternating current through the voice coil, a force is induced and the diaphragm can be caused to vibrate and, therefore, radiate sound waves.

[0003] Sometimes undesirably, the forces induced in the voice coil also induce unexpected reaction forces on the motor system, following Newton's third law of motion. The mechanical vibrations caused by the reaction forces on the motor are transmitted via the driver chassis and can excite the walls of the loudspeaker enclosure; in many loudspeaker systems, this form of excitation is the primary cause of motion in the enclosure walls. Since the walls have a large area and exhibit structural resonances, they can radiate significant sound, resulting in tonally distorted output from the loudspeaker.

[0004] Various solutions have been proposed to avoid this magnet vibration. U.S. Patent No. 4,805,221 is one of several patents that disclose a loudspeaker having two substantially identical diaphragm and drive assemblies mounted back to back. The permanent magnets of each assembly are rigidly coupled together by a tie rod so that any reaction force in one magnet is counteracted by an opposing reaction force in the other magnet. In this way, magnet vibration is reduced, along with the corresponding sound radiation from the housing wall. Our own British Patent No. GB2491108 shows another approach using back-to-back drive assemblies.

[0005] Since it is necessary to ensure that, in use, the reciprocating components do not affect any static components (which would seriously degrade the sound quality), the total thickness of the back-to-back loudspeaker is greater than twice the axial thickness of each individual drive assembly. One way to make such an arrangement significantly more compact in the axial direction is to coaxially integrate the two loudspeaker voice coil drivers to "nest" the two motor structures in what is referred to herein as a mirrored coaxial array. Mirrored coaxial array loudspeakers are commonly used in mobile phones and headsets, where the quality of the sound reproduction is not as important as the compact size; in such an arrangement, the maximum excursion of the voice coil (the distance between the farthest positions the voice coil can take away from its relaxed position in use) is limited, thereby maintaining the compact thickness of the loudspeaker. The axial compactness of the mirrored coaxial array does not allow the same degree of reaction force cancellation or vibration cancellation as in a back-to-back design, so that the sound quality from such an arrangement is significantly impaired compared to what is possible with a back-to-back design.

[0006] As an example of a mirrored coaxial array, European patent application No. EP1257147 discloses a speaker for a mobile phone, which includes: a first magnet; a second magnet provided to surround the first magnet; a yoke for connecting the first magnet and the second magnet; a first voice coil; a second voice coil; a first diaphragm connected to the first voice coil; a second diaphragm provided opposite to the first diaphragm relative to the first magnet and connected to the second voice coil; a first magnetic plate provided between the first diaphragm and the first magnet; and a second magnetic plate provided between the second diaphragm and the first magnet. The first voice coil is provided in a first magnetic gap between the first magnetic plate and the yoke. The second voice coil is provided in a second magnetic gap between the second magnetic plate and the yoke. This design is specifically designed so that the maximum voice coil excursion is small and generally the same for each coil, so that the speaker can be thin in the axial direction and suitable for use in a mobile phone. The magnetic circuit is arranged so that the magnetic flux in the magnetic gap flows in opposite directions; this is to maximize the driving force on the voice coil and provide sufficient driving force within the constraints of limiting the thickness of the arrangement. The maximum excursion of the voice coil is constrained by the need to keep the loudspeaker as thin as possible, so as to fit within the thin casing of a mobile phone (in the present invention, "excursion" is the movement of the voice coil in the axial direction as it reciprocates, and the maximum excursion defines the limit of the reciprocating motion; the maximum positive excursion is when the driven diaphragms are at their furthest separation, and the maximum negative excursion is when the diaphragms are at their closest). As mentioned above, designs that minimize the thickness of the loudspeaker (e.g., those in EP1257147) significantly compromise the quality of the sound reproduction. Summary of the invention

[0007] The present invention is based on the recognition that if the thickness of a mirrored coaxial array is compromised and the maximum excursion is increased beyond that which is feasible with known designs, it is possible to design a loudspeaker which remains relatively compact but is capable of higher quality sound reproduction than before and still allows for counterforce cancellation or vibration cancellation. Accordingly, the present invention provides a loudspeaker comprising: two acoustic diaphragms mounted to face in axially opposite directions; two voice coils, each having an axis and an axial length, and each configured to reciprocate along its axis to drive one of the diaphragms, the axes being substantially parallel and the two axes passing through the two diaphragms; and at least one magnet forming part of a chassis assembly, the chassis assembly being configured to provide two axially extending gaps for reciprocating each of the voice coils within one of the gaps, wherein the at least one magnet and the chassis assembly are adapted to cause magnetic flux to flow across the gaps in opposite directions, and wherein the voice coils overlap in the axial direction by between 10% and 90% of their average axial length when the diaphragms are at their predetermined maximum negative excursion in use, and wherein the voice coils do not overlap in the axial direction when the diaphragms are in a relaxed position between their maximum negative excursion and maximum positive excursion in use.

[0008] With such an arrangement, the voice coils overlap axially to a large extent at maximum negative excursion in use, but a corollary of this is that the chassis assembly (yoke and magnets) needs to be thicker in the axial direction to accommodate the increased movement of the voice coils towards each other and to maintain the necessary axial clearance, effectively increasing the axial thickness. A significant advantage is that it is possible to apply known reaction force cancellation and / or vibration cancellation techniques, thereby improving the sound quality compared to known mirrored coaxial arrays. It is possible to ensure that the force ("BL") generated by the drive system for each unit current flowing in the voice coil is constant when the voice coil is completely inside the magnetic gap. The voice coils must carry current in the same orientation in order to generate forces pushing in opposite directions. This is due to the fact that in each of the two magnetic gaps the magnetic field is radial, but in opposite directions. It would be intuitive to assume that this would lead to total inductance problems, as the coils would be very significantly coupled (and have very significant mutual inductance), but as will be explained, in practice these are not problems in practice. Since the axes of the two voice coils pass through the two diaphragms, this means that the voice coils are "nested" so that one voice coil reciprocates inside the perimeter of the other voice coil (i.e., viewed in the axial direction, the perimeter of one voice coil is completely within the perimeter of the other voice coil). The magnetic circuit in this mirrored coaxial array has two gaps and therefore has a relatively higher magnetic resistance than a conventional motor circuit, and this helps reduce the efficiency of the chassis assembly (usually a steel yoke) for amplifying the coil inductance.

[0009] The overlap of the voice coils at maximum negative excursion may be greater than 25%, preferably greater than 50%. If the voice coils are coaxial, the radial forces between them are more likely to be balanced, and the design process is easier. The voice coils may have the same axial length, or one may be longer than the other - although the reciprocating masses are preferably substantially the same: where one voice coil is smaller than the other so as to fit therewith, the masses are equalised by adding mass to one of the voice coil / diaphragm assemblies (which in most cases will be done for arrangements with inner voice coils).

[0010] The chassis may further include a cylindrical spacer shaped to extend axially and positioned to separate the two axially extending magnetic gaps. Preferably, the spacer is formed of a non-magnetic material (e.g., aluminum). The spacer is surprisingly advantageous because it addresses the inductive effects that would otherwise be caused by voice coil coupling; in use, eddy currents are generated in the cylindrical aluminum spacer, which reduces the self-inductance and mutual inductance of the voice coil, particularly when the coil is displaced rearwardly and immersed in the chassis assembly.

[0011] There may be a single magnet which may be annular and surround the magnetic gap, or have one magnetic gap inside and one magnetic gap outside, or the magnet may be a disc magnet with two magnetic gaps outside the magnet. Alternatively, there may be both a disc magnet and an annular magnet surrounding it, in which case the magnetic gap will be sandwiched between the two magnets. The chassis assembly preferably includes a yoke and / or end plates made of a magnetic material (e.g. steel) to complete the magnetic circuit. It will be understood that the loudspeaker is adapted so that the voice coils do not overlap in the axial direction when the diaphragm moves between its relaxed position and its predetermined maximum positive excursion in use.

[0012] In the movement of each voice coil between the maximum negative and maximum positive excursions of its associated diaphragm, the relaxed (or "rest") position of the voice coil will typically be midway between the maximum negative and maximum positive excursions of the diaphragm. In use, the movement of the voice coils is synchronized in opposite directions, preferably so that the diaphragm and voice coil reach their maximum positive and maximum negative excursions simultaneously. The movement of the voice coils in use may cause the voice coils to pass through their relaxed positions simultaneously.

[0013] If the movement of the voice coil from its relaxed (or "rest") position to its maximum negative excursion is characterized by a movement from 0% to 100%, then this range of movement during which there is no axial overlap of the coils is preferably 0-50%, more preferably 0-30%, even more preferably 0-20%; in other words, the axial positions of the inner ends of the voice coils coincide, and the axial overlap begins at the 50% or 30% or 20% point in the total range of movement of the voice coil between the "rest" position and its maximum negative excursion.

[0014] For simplicity, the invention is described primarily with reference to a circular voice coil (in the form of a substantially planar ring with a central hole); however, the invention is equally applicable to non-circular arrangements, for example, oval, elliptical or racetrack shaped (figure 8, or triangular / square / polygonal with rounded corners) voice coils or any shape that is symmetrical in one or two orthogonal directions lying in a general plane perpendicular to the voice coil axis and having a central hole. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will now be described by way of example and with reference to the accompanying drawings, in which:

[0016] Figure 1 is a schematic cross-sectional view of a conventional ring magnet loudspeaker drive unit;

[0017] Figure 2 is a graph showing approximate motor strength relative to voice coil displacement;

[0018] Figure 3 is a cross-sectional schematic diagram of an embodiment of a mirrored coaxial array according to the present invention;

[0019] Figure 4 yes Figure 3 Another view of the mirrored coaxial array in use and showing the maximum overlap of the voice coils, with the voice coils at maximum negative excursion, and

[0020] 5(a) to 5(c) are schematic illustrations of alternative embodiments of mirrored coaxial arrays. DETAILED DESCRIPTION

[0021] Figure 1 A conventional cantilevered ring magnet motor system 1 is shown. Typically, two of them are placed back to back when used in a reaction force cancellation / vibration cancellation arrangement (as described in US 2014 / 211963). A ring magnet 2 surrounds a steel yoke 4 in the form of a central cylinder 6 having front and rear end plates 8, 10. There is a magnetic gap formed by a circular hole 12 in the front end plate 8, and the hole 12 leads directly to an axially extending gap 14 between the magnet 2 and the cylindrical part 6 of the yoke 4. A voice coil 16 carrying a varying current reciprocates in the magnetic gap 12. The voice coil 16 is mounted to a diaphragm (not shown) at its outer end (the upper end as shown in the figure), and the reciprocating motion of the voice coil causes the diaphragm to vibrate, generating sound waves, as is well known in the art. In use, the voice coil moves between a negative excursion into the aperture 12 (in the figure, when the voice coil is displaced downwards) and a positive excursion out of the aperture 12 (in the figure, when the voice coil is displaced upwards).

[0022] The coil length L1 and the thickness of the plate determine the maximum deflection of the motor system. When the coil is completely inside the gap, the force (BL) produced by the motor system for each unit current flowing in the coil is constant. When the coil is deflected by ½ L1, BL will drop to about 50%, and this is usually the approximate maximum deflection (E1) of the motor system.

[0023] The total motor system height (H1) is

[0024] H1 = BP1 + FP1 + C1.

[0025] C1 is the distance between the gap and the yoke into which the voice coil can move during use. Figure 2 , if the voice coil moves completely out of the gap, the motor strength drops to a value close to zero. However, in practice, the voice coil is connected to a dynamic mechanical system, and mechanical inertia can cause the voice coil to travel beyond this range. Therefore, it is common practice to build in some extra clearance margin to ensure that collision never occurs.

[0026] .

[0027] Higher clearance margins provide better assurance that collisions will not occur, but this comes at the expense of motor system compactness. Typical clearance margins are in the range of 10% to 50%, depending on the application of the loudspeaker driver and the required compactness.

[0028] Typically, the thickness (BP1) of the rear plate 10 and the thickness (FP1) of the front plate 8 will be the same or very close. This is because both plates carry magnetic flux in similar orientations and therefore will have similar saturation when they have the same thickness (balancing saturation with steel volume is a key aspect of motor system cost and performance optimization).

[0029] To summarize, for two actuators placed back to back, the total height of the two motor systems is approximately

[0030] .

[0031] Figure 3 An embodiment of a mirrored coaxial array 11 according to the present invention is shown. Figure 3 In the invention, two cylindrical voice coils 26, 28 of different diameters are coaxially positioned using a single annular magnet 22 to provide magnetic flux in two axially extending gaps 46, 48. The accompanying drawings show the voice coils 26, 28 in their "resting" or relaxed position, wherein there is no axial overlap. A non-ferrous cylindrical spacer 30 is provided to hold the steel yoke 34 in the correct position, and the spacer 30 also separates the two gaps 46, 48. The spacer 30 should be electrically conductive to reduce the inductance of the two voice coils 26, 28. In use, the voice coils 26, 28 reciprocate through the magnetic gaps 42, 44 in the front and rear end plates 38, 40, and at maximum positive excursion (when the two coils are axially separated the farthest, when the coils will be closer than the magnetic gaps 42, 44). Figure 3 ) and the maximum negative offset (when the two coils are axially closest together, as shown in Figure 4 Entering and leaving axial gaps 46, 48 between the two surfaces (as shown).

[0032] Typically, the goal is that the motor strength (BL) of both voice coils 26, 28 is the same, and the maximum excursion of both voice coils is also the same. Typically, the thickness FP1, FP2 of both end plates 38, 40 will be the same. Typically, the length L1, L2 of both coils 26, 28 will be the same. Under these conditions, the gap for both coils will be the same. Under these conditions, the total thickness (height in the figures) of the dual motor system 11 is

[0033]

[0034] That is, half the thickness / height of conventional motor systems.

[0035] like Figure 4As shown in , at maximum negative excursion the two coils 26, 28 will be displaced by approximately ½L1 + ½FP, and the coils overlap by a significant margin OL. This case is quite extreme, since with the coils in this position the motor strength is almost zero, but this can easily occur at high power input levels, and particularly due to the inertia of the moving parts of the loudspeaker driver.

[0036] Assuming that the coil lengths L1, L2 are the same and the end plate thicknesses FP1, FP2 are the same, the overlap (OL) at this coil position is given by

[0037] .

[0038] From this, it is apparent that the overlap can be expressed as a percentage of the voice coil length

[0039] .

[0040] Given typical air gap margins, the maximum voice coil overlap is between 50% and 90%.

[0041] Since one magnet ring 22 is used for both magnetic gaps 42, 44, it is generally necessary to use a larger number of magnets 22 than in a typical single motor system. In some cases, this may mean that the gap margin is larger than normal to allow the thickness of the magnet ring 22 to be as large as possible. Obviously, this is a balance between motor-system strength and motor-system thickness that the designer must fine-tune. Even in this case, the maximum overlap of the coils will be significant and may be at least 10%, and may be greater than 25%.

[0042] The magnetic fields in the two magnetic gaps 42, 44 are oppositely oriented. Typically, this motor system would be required to deliver the same force on both coils 26, 28 but in opposite directions in order to form a "reaction cancelling" arrangement and would therefore necessitate connecting one of the coils in the reversed direction.

[0043] Advantageously, both coils 26, 28 have the same motor strength. This is relatively easy to achieve because the two magnetic gaps 42, 44 are in a series magnetic connection and the same magnetic field passes through both. Since approximately the same magnetic flux passes radially through both magnetic gaps 42, 44, the magnetic flux density in each magnetic gap is approximately proportional to the voice coil diameter. Therefore, the flux density experienced by the smaller diameter voice coil 28 will be higher. However, this effect is balanced by the smaller coil circumference of the smaller diameter voice coil 28, and therefore, it is fairly easy to achieve approximately the same motor strength BL on both coils 26, 28 (especially since there are many geometric and coil parameters that can be adjusted to minimize the difference).

[0044] In some cases, it may not be possible or desirable to achieve the same motor strength. In this case, it may be advantageous to drive the two coils 26, 28 with different signals so as to still achieve approximate reaction force cancellation.

[0045] It is possible that this motor system arrangement may have advantages when not used in a reaction force cancellation mode (where no specific relationship exists between the signals in the two coils). In this case, the compactness and overlap of the voice coils may still be advantageous.

[0046] Fig. 5(a) shows an alternative embodiment in which the two coils 56, 58 are separated by a spacer 50, as in the previous embodiment, but a disk 52 made of magnetic material is located inside the smaller coil 58. Fig. 5(b) shows another embodiment in which the two magnets 62, 64 are separated by a spacer 60. Fig. 5(c) shows a less useful modification in which a single ring magnet 72 is located between the two coils 76, 78; this form is even less useful because the difference in coil diameters must be greater than in the previous embodiment in order to leave room for the ring magnet 72, because in other embodiments the aluminum spacer helps minimize voice coil inductance and reduce distortion, but is not used in this embodiment, and because the two gaps 82, 84 are now positioned magnetically parallel, it may be more difficult to achieve the same magnetic flux in the two gaps.

[0047] Of course, it will be appreciated that many variations may be made to the embodiments described above without departing from the scope of the invention. For example, the central cylindrical portion of the yoke may be solid (e.g. Figure 3 ), or with an axial hole (as shown in Figure 1 ). The yoke is described as being made of steel, but any ferromagnetic material may be used, and the spacer is described as being made of aluminium, but any non-magnetic conductive metal or alloy may be used. The magnets may be of any suitable type or manufacture; the spacer may be a solid cylinder, it may be formed from sections that fit together, and / or it may have an axially extending aperture. As we described in our earlier application GB2567673, the axially extending gap may contain sound absorbing material (e.g. acoustic foam, fabric, open and closed cell foam or other porous material) to reduce resonance.

[0048] Where various variations or alternative arrangements have been described above, it will be appreciated that embodiments of the invention may incorporate such variations and / or alternatives in any suitable combination.

Claims

1. Speakers, including: two acoustic diaphragms, mounted to face in axially opposite directions; two voice coils, each having an axis and an axial length, and configured to reciprocate along its axis to drive one of the diaphragms, the two axes being substantially parallel and the two axes passing through the two diaphragms; and at least one magnet forming part of a chassis assembly, the chassis assembly being configured to provide two axially extending magnetic gaps for reciprocating each of the voice coils within one magnetic gap, wherein the at least one magnet and the chassis assembly are adapted to cause magnetic flux to flow in opposite directions across the magnetic gaps, wherein the chassis assembly further comprises a spacer formed of a non-magnetic conductive material, the spacer being shaped to extend axially and positioned to separate the two axially extending magnetic gaps, and wherein, when in use the diaphragm is at its predetermined maximum negative excursion, the two voice coils overlap in the axial direction by between 10% and 90% of their average axial length, and wherein, when in use the diaphragm is in a relaxed position between its maximum negative excursion and maximum positive excursion, the two voice coils do not overlap in the axial direction.

2. The loudspeaker according to claim 1, wherein The overlap is greater than 25%.

3. The loudspeaker according to claim 1, wherein The overlap is greater than 50%.

4. The loudspeaker according to claim 1, wherein: The two axes are coaxial.

5. The loudspeaker according to claim 1, wherein The two voice coils have the same axial length.

6. The loudspeaker according to claim 1, wherein The mass of the diaphragm and its associated voice coil facing one direction is substantially the same as the mass of the diaphragm and its associated voice coil facing the other direction.

7. The loudspeaker of claim 1, comprising a single magnet.

8. The loudspeaker according to claim 7, wherein: The magnet is formed as a closed loop and extends axially so as to surround the two axially extending magnetic gaps.

9. The loudspeaker according to claim 7, wherein: The magnet extends axially and is surrounded by the two axially extending magnetic gaps.

10. The loudspeaker of claim 1, comprising at least two magnets.

11. The loudspeaker according to claim 1, wherein The chassis assembly includes a yoke.

12. The loudspeaker according to claim 1, wherein When the diaphragm moves in use between its relaxed position and its predetermined maximum negative deflection, for the first 50% of this movement the two voice coils do not overlap in the axial direction.

13. The loudspeaker according to claim 12, wherein: For the first 30% of the movement of the two voice coils between the relaxed position and a predetermined maximum negative excursion of the two voice coils, the two voice coils do not overlap in the axial direction.

14. The loudspeaker according to claim 1, wherein The relaxed position of one or both voice coils is located midway between the maximum negative excursion and the maximum positive excursion of the one or both voice coils.

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